Polysilicon has been used in semiconductor devices due to its thermal resistive properties during high temperature processes, which allows it to be annealed at high temperatures along with source/drain structures and to block the channel region from doping atoms and form self aligned source/drain structures. As technology nodes shrink, there has been a desire to replace a gate electrode made of polysilicon with a metal gate electrode to improve device performance with the decreased feature size.
A gate last process may be implemented to replace the polysilicon gate electrode with a metal gate electrode. The gate last process refers to the metal gate structure being formed after the formation of the source/drain structures. However, problems arise when replacing the polysilicon gate electrode with a metal gate electrode. Since a high amount of etchant is applied to remove the polysilicon to prevent residues, the etchant could penetrate through weak points of the sidewall of the gate structure and damage the underlying features such as the source and drain regions. Therefore, the damage issue to the underlying features needs to be addressed to obtain the desired device performance.